High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR)

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Azimuth Ambiguity Resolution Workshop

Introduction

Bruce W. Lites

High Altitude Observatory

National Center for Atmospheric Research

CSAC/HMI Workshop, Boulder, 26-27 September 2005

26 September 2005

Azimuth Ambiguity Fundamentals

Bruce Lites, CSAC/HMI Ambiguity Resolution Workshop, September 2005

Zeeman Effect produces linear polarization for a transverse magnetic field:

Note the degeneracy of the polarization wrt the sense of the transverse field (up or down in this case).

•The “local solar frame” (one axis normal to the solar surface) is the preferred reference frame for physical interpretation

•For fields measured away from disk center, the ambiguity of the azimuth in the observer’s frame affects both the local frame azimuth and inclination to the vertical

•The azimuth ambiguity must be resolved for many problems that depend on vector field measurements:

•Extrapolation of fields to higher layers

•Flux emergence and decay

•Active region evolution

•…………

Why We Concern Ourselves with the Azimuth Ambiguity

Ambiguity resolution necessitates adding more physical information to that provided by the Zeeman Effect alone.

Additional information on the physical system:

• Field connectivity (H, X-ray, EUV, ….)

• Solenoidal condition B = 0

• Scattering polarization (in chromospheric lines)

• ….Others (add to the list at this workshop!)

Ad-hoc physical assumptions:

• Closest approach to potential field

• Force-Free field assumption (currents parallel to B)

• Minimize vertical currents (spatial continuity)

• Assumptions on physical structure of field topology (e.g., plage fields usually close to vertical, orientation gradients at location of strong field gradients, …..)

Ambiguity Resolution – “There Is No Magic Bullet”

• As of yet, none of the class of additional physical information provides us with a reliable ambiguity resolution

• Some ad-hoc physical assumptions do better, but still fail in particular circumstances

• Ad-hoc assumptions can be patently wrong (e.g., the assumption of a force-free field in photosphere is often incorrect)

•Data quality and analysis techniques strongly affect the local frame azimuth, hence the ambiguity resolution

•Manual (human interaction) adjustment is sometimes required for any extant method (what are the assumptions involved?)

Workshop Goals (Personal View)

• Further our collective understanding of the methods that are available

• Examine the strengths and weaknesses of the methods

• Compare, to the extent possible, the accuracy and usefulness of available methods

• Assess the utility of various methods for bulk, automatic processing of data from HMI, Solar-B, SOLIS, …..

• Discuss possibilities for future advances

• Recommend a course for further study (Another workshop? Working groups?)

SDO/HMI - CSAC Azimuth Ambiguity Resolution Workshop Agenda NCAR CG-1 Room 2126

26-27 September 2005 Monday 26 September 2005

8:30 Coffee, Continental Breakfast Items

9:00 Welcome: Michael Knoelker, Director HAO

9:05 Introduction to the Workshop: Bruce Lites

SUMMARIES OF AMBIGUITY RESOLUTION METHODS

9:15 Allen Gary: Ambiguity Resolution at the MSFC Magnetograph

9:35 K.D. Leka: Hawaii Iterative Method

9:55 Tom Metcalf: Simulated Anealing

10:15 Break

10:40 Bruce Lites: The AZAM Utility

11:00 Manolis Georgoulis: Structure Minimization and Nonpotential Magnetic Field Calculation (NPFC)

11:30 Alexei Pevstov (presented by K.S. Balasubramaniam): New Iterative Method of the Azimuth Ambiguity Resolution

11:50 Additional presentations by teleconference, if necessary

12:30 - 13:30 Lunch in CG-1 Cafeteria

13:40 Discussion: How Do We Compare Ambiguity Resolution Methods?

14:20 Comparison of Ambiguity Resolution Methods: Model Data

15:30 Break

15:45 Comparison of Ambiguity Resolution Methods: Model Data (continued).

17:30 Adjourn

19:30 Dinner at Selected Boulder Restaurant

SDO/HMI - CSAC Azimuth Ambiguity Resolution Workshop Agenda

Tuesday 27 September 2005 8:30 Coffee, Continental Breakfast Items 9:00 Comparison of Ambiguity Resolution Methods: Application to Observations 10:30 Break 10:50 Discussion: Ambiguity Resolution Methods for Automatic Processing

(Discussion Leader, Jack Harvey) Which extant method(s) now seem best for continuous large data sets from the

likes of SOLIS/VSM, Solar-B, HMI? 12:20 - 13:30 Lunch in CG-1 Cafeteria 13:40 Discussion: Possible New Avenues for Ambiguity Resolution

*(Discussion Leader, K.S. Balasubramaniam) 15:00 Break 15:20 Tom Metcalf: Workshop Summary and Final Discussion 16:00 Adjourn

Note: Agenda purposefully left open to allow ample time for discussions and comparisons.

Some Logistical Information:

• Proceedings: We will post all presentations and summaries on the web. Please give your presentations to Steve Tomczyk.

• Money: Those of you being reimbursed from NCAR for travel expenses, please see Louise Beierle at this morning’s coffee break. She will have travel vouchers, etc. for you to sign.

• Dinner tonight: We will break into smaller groups for dinner at various restaurants. We will meet in front of the Residence Inn at 19:00 to arrange transportation and select restaurants.